Tuneable laser

ABSTRACT

A tuneable laser having an active section, a phase section and a Bragg reflector comprising a plurality of discrete grating units, at least two of which gratings have a different pitch, wherein current is applicable to at least the grating having a longer pitch, such that the effective wavelength of the grating having a longer pitch can be tuned to the wavelength of the grating having a shorter pitch. A chirp grating can provide the difference in pitch.

RELATED APPLICATIONS

This application is a 35 U.S.C. 371 national stage filing ofInternational Application No. PCT/GB02/01329, filed 19 Mar. 2002, whichclaims priority to Great Britain Patent Application No. 0106790.9 filedon 19 Mar. 2001, and Great Britain Patent Application No. 0114970.7filed on 19 Jun. 2001 in Great Britain. The contents of theaforementioned applications are hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to a tuneable laser, in particular but notexclusively to a three section distributed Bragg reflector tuneablelaser.

BACKGROUND OF THE INVENTION

Tuneable lasers are well known in the field of optical communications,particularly in connection with wavelength division multiplextelecommunication systems, which rely upon either being fed by stacks ofindividually wavelength distributed Bragg reflectors (DBR) lasers, whichcan be individually selected, or by a wide tuning range tuneable laserthat can be electronically driven to provide the wavelength required.Limited tuning range tuneable lasers that rely upon thermal effects fortuning are also available.

In this specification the term “light” will be used in the sense that itis used in optical systems to mean not just visible light but alsoelectromagnetic radiation having a wavelength between 1000 nanometers(nm) and 3000 nm.

U.S. Pat. No. 4,896,325 discloses a wavelength tuneable laser havingsampled gratings at the front and rear of its gain region. The gratingsproduce slightly different reflection combs which provide feedback intothe device. The gratings can be current tuned in wavelength with respectto each other. Co-incidence of a maximum from each of the front and reargratings is referred to as a supermode. To switch the device betweensupermodes requires a small electrical current into one of the gratingsto cause a different pair of maxima to co-incide in the manner of avernier. By applying different electrical currents to the two gratings,continuous tuning within a supermode can be achieved. In practice, thereflection spectra of the known sampled grating structures have a sinesquared envelope which limits the total optical bandwidth over which thelaser can reliably operate as a single mode device.

In contrast to the Segmented Grating Distributed Bragg Reflector(SG-DBR) described above, a Phase Shift Grating Distributed BraggReflector (PSG-DBR) is disclosed in GB-A-2331135. This has a pluralityof repeat grating units in which each grating unit comprises a series ofadjacent gratings having the same pitch, which gratings are separated bya phase change of π radians, wherein the gratings have different lengthsto provide a pre-determined reflection spectrum.

The known devices have Bragg gratings which bound both ends of the gainand phase regions of a four section tuneable laser, which produces acomb wavelength response. For a given set of drive currents to the frontand rear grating sections there is simultaneous correspondence inreflection peak at only one wavelength, as a consequence of which thedevice lases at that wavelength. To change this wavelength a differentcurrent is applied to the front and rear gratings. Thus the front andrear gratings operate in a vernier mode, in which the wavelengths ofcorrespondence determine a supermode wavelength. Although the knowndevices have generally been acceptable, they share a tendency to sufferfrom short wavelength losses, which in combination with the frontgrating tuning absorption reduces the output power of the laser.

U.S. Pat. No. 5,379,318 discloses a DBR laser with an extended tuningrange. A plurality of discrete gratings are arranged on opposing sidesof a gain section. One discrete grating on each side is tuned so thateach exhibits a wavelength-specific reflectivity which co-incides withthe other, and thus the gain section is caused to lase at thatwavelength. Discrete gratings are however required on both sides of thegain region.

BRIEF DESCRIPTION OF THE INVENTION

The present invention seeks to provide a tuneable laser with a higheroptical output power whilst having acceptable manufacturing costs.

By the present invention there is provided a tuneable laser having aphase change section bounded on one side by a Bragg reflector and on theother side by a gain section, the Bragg reflector reflecting at aplurality of wavelengths and being capable of having current passingselectively through discrete sections so that one or more portions ofthe Bragg reflector can be tuned together with a portion reflecting at alower wavelength to reflect at that lower wavelength.

The Bragg reflector may be a chirp grating and may be formed in amaterial having a refractive index variable in response to the passageof current therethrough, there being a plurality of external electrodesalong the length of the grating, with each electrode being selectivelyconnectable to a power source.

According to a further aspect of the invention there is provided atuneable laser having an active section, a phase section and a Braggreflector comprising a plurality of discrete grating units, at least twoof which gratings have a different pitch, wherein current is applicableto at least the grating having a longer pitch, such that the wavelengthof the grating having the longer pitch can be tuned to the wavelength ofthe grating having a shorter pitch.

In use, current is applied to the grating unit having a longer pitch sothat it is optically equivalent to the adjacent grating having a shorterpitch when the respective reflective maxima superpose, thereby providingthe dominant wavelength at which the device can lase. This lasingwavelength can then also be current tuned in the known manner.

Preferably, the tuneable laser is provided with a simple partialreflecting front mirror. In a preferred embodiment, the reflectorcomprises a plurality of discrete grating units, each having a constantrespective pitch corresponding to respective predetermined wavelengths,the grating pitch increasing with distance from the active section.Preferably each grating unit has an independently actuable electrode.Preferably a conventional switching circuit is provided to switch thecurrent to the electrodes and grating units.

The tuneable laser of the invention has a number of advantages over theknown designs, in particular minimising the short wavelength lossesinherent in the four section DBR lasers of the prior art, thereby havinghigher power output by dispensing with the front Bragg reflector,absorption is minimised as there is no contribution to tuning inducedabsorption from the front Bragg reflector, which usually dominatesabsorption. Also the absorption losses due to tuning in the Braggreflector may be less than in known SG-DBR lasers as the use of ashorter grating is facilitated.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described withreference to the drawings which show;

FIG. 1 shows a schematic representation of a three-section laser,

FIG. 2 shows a schematic representation of a Bragg reflector,

FIG. 3 a shows a wavelength envelope using a known Bragg reflector,

FIG. 3 b shows a wavelength envelope using the Bragg reflector of theinvention,

FIG. 4 shows a second embodiment of the invention

FIG. 5 shows a chirp grating

FIG. 6 shows a chirp line

FIG. 7 shows an enlarged view of light passing down a chirp grating

FIGS. 8 and 9 shows box diagrams of light intensity vs wavelength

FIG. 10 is an explanatory view of a chirp laser chirp line and lightintensity, and

FIGS. 11 and 12 show chirp line drawings.

DESCRIPTION OF THE INVENTION

The wavelengths of interest referred to above, for example the C-bandwavelengths of 1530 to 1570 nm are the wavelengths of light in freespace. When such light passes through a medium, of refractive indexn_(eff) the wavelength of the light changes. The actual wavelength ofthe light within that medium which will be referred to herein as λ′, isthe wavelength λ divided by the value for the refractive index n_(eff).In other wordsλ′=λ/n _(eff).where n_(eff) is the effective refractive index of the medium as seen bythe propagating light.

It so happens that the glass (silica) fibres, which are commonly used intelecommunications systems, have low loss regions at about 1100 nm 1300nm and 1500 nm. These regions are about 100 nm wide and consequentlymuch work is done on producing lasers that produce light in the low lossregions. The same is true for the tuneable laser of the presentinvention. The specific examples of the invention are designed to workin the C-Band, but the invention could be used for other wavelengths ifrequired and if new types of fibre optical cables become available.

FIG. 1 shows a three section tuneable DBR semiconductor laser having anactive or gain section 1, a phase section 2 and a rear mirror section 3.Both the gain section 1 and the phase section 2 are provided withelectrodes 4, 5. At the boundary of the gain section 1, a front mirror20 is provided, which mirror can be a simple partial reflecting mirroror other suitable mirror. The laser further comprises an active region6, the Bragg reflector layer 7 and substrate layer 8.

The rear mirror section 3 has a plurality of discrete Bragg gratingunits 10–16 etched into the waveguide. The pitch of the respectivegrating units increases with distance from the active section, with thegrating having the shortest pitch closest to the active section and thegrating with the longest pitch furthest from the active section. Each ofthe gratings 10–16 has an associated electrode 18 which electrodes canbe actuated independently of one another.

In use in, for example, a C-Band or L-Band 40 nm optical communicationsystem, the difference in the wavelength between adjacent grating units10–16 will typically be about 4 nm and the pitch of the respectivegrating unit 10–16 can be determined by the Bragg conditionλ′=2Λwhere λ′ is wavelength, and Λ is the pitch for first order gratings,which are preferred as they provide the strongest coupling with no outcoupling loss.

FIG. 2 shows a schematic representation of a Bragg reflector forming therear mirror section 3. The Bragg reflector comprises a plurality ofdiscrete grating units, in this case eleven for a 40 nm bandwidth, ofwhich seven are illustrated, 10–16, each having a different pitch. Thegrating unit closest to the phase section 2 has the shortest pitch andthe pitch of each successive grating unit remote from the phase sectionis greater than the pitch of the preceding unit. Each of the gratingunits 10–16 has an associated electrode 18, wherein each electrode canbe actuated independently of one another. The Bragg grating can befabricated using electron beam writing techniques or phase maskholographic techniques.

In use, the arrangement of grating units produces a reflection spectrumhaving a series of comparatively small maxima, which are typicallycloser together than in the prior art systems. If a current is appliedto one of the grating units 10–16, the effective refractive index of thegrating and the active material immediately underneath the electrode isdecreased and hence the wavelength of the grating can be tuned. Byappropriate tuning it is possible to tune one of the maximacorresponding to one of the grating units until it blends with theadjacent maximum of the grating having a shorter pitch to form adominant wavelength, at which the device will start to lase. Bysuccessively tuning the maxima to blend with the corresponding shorter‘wavelength adjacent maxima’ the laser can be tuned across a widebandwidth. Typically, the grating pitches will be chosen so that therespective reflective peaks are separated by approximately half themaximum single peak tuning. e.g. 4 nm. Therefore, the first wavelengthis tuned until it blends with the adjacent wavelength (second), lasesand then this wavelength can be current tuned until it reaches the nextgrating wavelength (third), at which point the current to first gratingis cut off, and wavelength tuning continues with current drive in thesecond and third grating units.

FIG. 3 a shows schematically part of the typical reflection spectrumobtained when no current is applied to the electrodes. The reflector hasdiscrete reflective peaks shown which are of substantially similarintensities. These wavelength peaks are generally separated by about 4nm for a C-Band or L-Band device.

When current is applied to an electrode in the manner described inrelation to FIG. 2 and so that one of the gratings, e.g. 55, is tuned,ultimately a spectrum is obtained as shown in FIG. 3 b. In this case,the peak 54 has an intensity approximately double that of each of thepeaks 51, 52, 53 and 56. It is at the wavelength peak 54 that the devicewill lase. By design the peaks will be substantially in phase when theyblend together and thus reinforce however, it is possible that phasingeffects generate features on the particular reflection maxima createdwhich could be useful in particular applications such as to avoid modehopping.

The laser cavity length will vary in dependence on the specific Braggwavelength selected. In order to avoid mode hopping, and keep a constantoptical cavity length, the phase section 2 is driven with current, seeFIG. 1. It would, however, be possible to control the phase byselectively applying current to the electrodes located between thelowest grating used for wavelength selection and the gain section.

Although the front mirror is preferably a simple partial reflectingmirror, it can be any suitable mirror which will have the samereflection wavelength response as the rear mirror. Whilst a simplemirror will minimise losses, a Bragg grating could be used.

The invention design may be suitably applied to solid-state lasersmanufactured using Group III–V or other semiconductor materials.

The photoluminescent gain curve of semiconductor materials is curvedwith intensity falloff at the edges of the spectrum. To produce auniform intensity gain response across the bandwidth of interest theBragg grating unit length can be varied to give enhanced reflectivitywhere required.

The reflectivity profile of the grating can be controlled such that thereflection peak has a sharp definition at the wavelength of interestthereby giving good side mode suppression. An example of such a gratingis one that has along its length a partial or complete, or invertedGaussian or Lorentzian reflectivity profile.

Referring to FIG. 4, this shows an alternative form of the invention inwhich the discrete Bragg gratings are replaced with a chirp grating. Thelaser assembly includes a gain section 60 a phase change section 61 anda chirp grating section 62. Located on the gain section is an electrode63 to enable the passage of current into the gain section. Located onthe phase change section is an electrode 64 to enable the passage ofcurrent into the phase change section and located on the chirp Braggsection is a series of individually selectable electrodes 65 to 72 topermit current to be passed selectively into portions of the chirpgrating within section 62.

The chirp grating is a form of Bragg grating which has a substantiallycontinuous variation in the wavelength at which it reflects light alongits length. It is thus distinguished from a normal DBR which reflects ata single wavelength and also from a sampled grating DBR as shown inFIGS. 1 and 2, which reflects at a plurality of discrete wavelengths atdifferent positions along its length.

A chirp grating is formed at the interface between two materials ofdifferent refractive index and can be represented graphically as asinusoidal shaped waveform, or as a castellated form. The physical shapeof the grating is dependant upon the etching technique employed and mayresult in a castellated form, particularly when a dry etching process isused to produce the grating, e.g. reactive ion etching.

The refractive index, n, of the material used in the production of thechirp grating through which the majority of the light passes is aquarternary material (InGaAsP) and the refractive index of the materialvaries with the wavelength of light passing through the material.Typically n at 1570 nm is 3.33, at 1550 nm n is 3.38 and at 1530 nm n is3.43. Thus n decreases by about 3% from 1530 nm to 1570 nm.

An explanation of the construction and operation of the chirp grating isprovided by FIGS. 4 to 12.

As shown in FIG. 5 the grating is formed as an interface 80 between theupper layer of material 81 of a low refractive index and a lower layer82 of a higher refractive index. This Interface can be represented as awaveform and the pitch Λ of the waveform making up the grating isgradually increased along the length of the grating from Λ_(S) at theshort end of the chirp grating to Λ_(L) at the long end of the grating.In FIG. 5 the increase in pitch is deliberately exaggerated todemonstrate what is happening. In practice the increase in pitch lengthover the whole of the grating is small, namely about 2.5%, so that atthe short end the grating reflects light of a wavelength of about 1530nm and at the long end the grating reflects light at a wavelength of1570 nm. Thus there is a 40 nm variation in the reflection wavelengthover the length of the grating, which is about 2.5% of the averagewavelength of 1550 nm.

In FIG. 6, there is a graph showing how the pitch of the grating variesalong its length with the pitch Λ in the vertical axis and the length ofthe grating x on the horizontal axis.

It will be appreciated that the pitch values, Λ, along the length of thegrating can be plotted directly against the length and a line isgenerated. The line can be straight or can be curved depending on howthe pitch length is varied along the length of the grating. If theincrease in grating pitch is at a constant rate the line is straight asshown at 83 a, and the grating is called a linear chirp grating. If theincrease in grating pitch along the grating is uniform, in other wordsin the direction of increasing Λ, each Λ is a certain small constantstep increment on the one before it, then the line will not be linearbut will curve downwards as shown at 83 b, as the line increasingly goesto the right.

Referring to FIG. 7, this demonstrates the effect of light passing alonga chirp grating. Again the grating is shown as a sinusoidal interface 84between an upper layer 85 of a lower refractive index and a lower layer86 of higher refractive index. The waveguide of the assembly of highrefractive index through which the majority of the light passes is shownat 87, separated from the lower layer 86 of the chirp grating by anintermediate layer 88 of low refractive index. Underneath the waveguide8 is a further low refractive index substrate 89. Superimposed on thelayer structure is a graphical representation of the wavefront of thelight passing through from left to right as at 90 in the direction ofthe arrows 91. Line 92 is an indication of the intensity of the light inthe layers of the assembly and it can be seen that most of the lightpasses through in the waveguide of high refractive index.

As shown in FIG. 7, the light passes not only through the waveguide butthe evanescent wave also passes along the layer 86 forming the lowerlayer of the chirp grating. If the light should happen to have awavelength λ′ which is twice the length of a pitch Λ then that lightwill be reflected back i.e. if λ′=2Λ then that wavelength of light willbe reflected. Thus the chirp grating as a whole will reflect light inthe range λ′_(S)=2Λ_(S) to λ′_(L)=2Λ_(L) where Λ_(S) is the shortestgrating pitch and Λ_(L) is the longest grating pitch. Light ofwavelengths outside of this range will not be reflected back along thewaveguide.

This can be represented diagrammatically as in FIGS. 8 and 9, which arebox diagrams of intensity of light I in the vertical axis and wavelengthλ′ in the horizontal axis. If a box of light of plurality of wavelengthsis admitted into the grating as shown at 93 in FIG. 8, the envelope iscomplete and represents all of the wavelengths between λ′₁ and λ′₂—whichare widely separated wavelengths. However as the chirp grating reflectscertain of the wavelengths, for example between λ′_(S) and λ′_(L) inFIG. 5, the emerging box of wavelengths 94 as shown in FIG. 9 has a gap95 which corresponds to those wavelengths between λ′_(S) and λ′_(L)reflected by the chirp grating.

The chirp grating will, if in its complete and unaltered condition,reflect all wavelengths between λ′_(S) and λ′_(L) without anypreference. However if one of the electrodes such as electrode 68 ofFIG. 10, has a current passed through it, then that will lower therefractive index of the material in which the chirp grating is created.This will result in the grating as a whole being selectively enhanced inits reflectivity at this specific wavelength and this can result in thelaser lasing at that wavelength.

This will be explained below in greater detail with reference to FIG.10. In this figure the upper portion shows the laser of FIG. 4. This ispositioned over the chirp diagram (as shown in FIG. 6) which in turn ispositioned over a drawing of the reflectivity of the chirp grating vsdistance.

It can be seen in the central portion of FIG. 10, where the gratingpitch Λ is plotted against distance x, that the chirp response line isshown by a line 96. It will be seen that line 96 has a region 98, showndotted below the main portion of the line, for reasons which will beexplained below.

On the outer surface of the laser there are a series of electrodes 63 to72. The electrode 63 can be used to inject current into the gain sectionto make it create light. The electrode 64 can be used to control thephase section as described below and the electrodes 65 to 72 are able toinject current into different regions of the grating 62.

If just sufficient current is injected into the gain section to make itgenerate light, then if the chirp section is capable of reflecting lightin the range of 1530 to 1570 nm the wavelengths of light within thatrange will be internally reflected. Light outside of the reflectingwavelengths will be absorbed or will be emitted from the ends of thelaser. The laser will not lase because the intensity of the light at allof the frequencies in the range 1530 to 1570 nm is below the lasingthreshold.

To get the laser to lase, it is necessary to have both a populationinversion of charge carriers within the gain material and to get atleast one, and preferably only one, wavelength to be above the lasingthreshold. This is achieved by injecting sufficient current into thegain section 60 through electrode 63 to create the population inversionand by making a portion of the rear grating reflect light of a specificwavelength preferentially, so that the rear grating selectively reflectslight of that particular wavelength. The front mirror will reflect backthe light of that wavelength, so that that wavelength will become thepreferred or enhanced wavelength and the laser will commence to lase atthat wavelength.

The selection of the particular wavelength is effected by passing acurrent through an electrode such as electrode 68 above the portion ofthe chirp grating which corresponds to the region 98 in the chirp curve96. The effect of the passage of current is to increase the currentdensity in that region of the grating, which lowers the refractive indexof the grating layer 86 just below the electrode 68. The lowering of therefractive index has the effect of making the grating reflect at a lowerwavelength, which is the same effect as would be obtained by shorteningthe grating pitches in that region.

This means that the effective grating pitches of the dotted portion 99as is shown in the central portion of FIG. 10 now line up with theadjacent region 97, forming a chirped Fabry-Perot étalon, which thusreinforces the reflection in the adjacent region 97.

Referring to the lowest portion of FIG. 10, which is a graph ofreflectivity η vs. distance x, it can be seen that there is a trough 98Ain the reflectivity of the grating which corresponds to the region 98that now reflects at a lower wavelength. However there is now anenhancement of the reflectivity of the region 97 due to the resonantchirped Fabry-Perot étalon structure. Thus there is produced thereinforced peak 99A in the reflectivity.

Light at the wavelength that corresponds to the position of peak 99A isthus selectively reflected. The front mirror as it reflects at allwavelengths reflects the light at the selected wavelength, and the lasercommences to lase at that wavelength.

It will be appreciated that without any further adjustments the lasercould only be tuned to as many different wavelengths as there areelectrodes 65 to 72.

However, the device can be made continuously tuneable if it is assumedthat the materials from which the chirp gratings are constructed have asufficiently variable refractive index.

FIG. 11 illustrates how this can be put into effect. In FIG. 11 there isshown the chirp grating which acts as a mirror under three differentconditions.

In the drawing there are shown ten electrode positions 100 to 109, whichcorrespond to the electrode positions 65 to 72 in FIG. 10. In otherwords, instead of there being eight electrodes over the rear grating,there are ten electrode positions in this schematic. The line 110corresponds to the line 96 of the grating as shown in FIG. 10. Thevertical dotted lines show the alignments of the electrodes and theportions of the chirp diagram.

In the upper portion of the FIG. 11 there is no current flowing throughany of the electrodes 100 to 109. The line 110 is continuous with noportion being preferred.

In the central portion of FIG. 11 a current is passed through electrode106. The current being half that required to cause the maximum reductionin the refractive index of the material of the chirp grating below theelectrode 106 which is equivalent to material 86 in FIG. 10. The resultof this is to displace downwards the portion 111 of the line 110. Thisresults in a selection of a particular wavelength at which the laser canlase in exactly the same manner as described above with reference toFIG. 10.

To further tune the laser, so as to reduce the wavelength at whichlasing is preferred, current is passed through all of electrodes 100 to105 and at the same time the current passing through electrode 106 isincreased. This causes a lowering of the portion 112 of the chirp linebelow its original position, shown dotted. The portion 111 a of the line110 also is lowered at the same time, thus moving the point of selectionto a lower wavelength. In best practice no additional current need bepassed through electrodes 107 to 109, as they play no part in thereflecting process. However, since they play no part in the selectionprocess, it is possible for the electrodes 107 to 109 to be lowered inamounts similar to electrodes 100 to 105 without interfering with thewavelength selectivity. When the current passing through the electrode106 is the maximum which can be applied to reduce n, and thus themaximum amount of fine tuning has occurred, the electrodes 100 to 105will be passing a current which corresponds to half of the totalreduction of n in the material in section 62 below electrodes 100 to105.

To further tune the laser, the current is removed from electrode 106 andis applied to the next adjacent electrode (or any other selectedelectrode) and the sequence of actions is repeated. By this means thelaser can be tuned over the entire 1530 nm to 1570 nm waveband.

The selectivity of the chirp at a particular wavelength can be enhancedas shown schematically in FIG. 12. This figure is similar to FIG. 11 butshows what happens when two adjacent sections of the chirp grating aremoved together.

In the upper portion of FIG. 12 the chirp grating is shown in the sameposition as in FIG. 11. This is also the case for the central portion ofFIG. 12, where current applied to electrode 106 has caused a lowering ofthe line 111 to the position half way down to its maximum extent. If thecurrent is passed through electrode 105 this causes the line 113 to belowered and the current passing through electrode 106 is increased atthe same rate so that lines 111 a and 113 move down in synchronism. Thismeans that the grating selectivity is increased by the enhancedreflectivity.

When the applied current to electrode 105 is half of that applied toelectrode 106 and the line 111 a is depressed to its maximum extent thelines 111 a and 113 will also coincide with portion 114 of line 110 togive a three-region coincidence.

It will be appreciated that the more electrodes that can be installedover the chirp, the greater the number of regions that can be broughtinto coincidence and the smaller each reduction in n needed at any pointto tune the laser. For a range of 40 nm total tuning, if say, twelvedifferent electrode positions were used, then each would only berequired to tune through a range of 4 nm to cover the entire band with adegree of wavelength margin at the band edges.

By this process, therefore, the laser can be tuned over the whole of thedesired wavelength range, depending on the bandwidth of the originalgratings.

During fine tuning the cavity length will vary and a phase changesection 61 controllable by electrode 64 is used to give a constantoptical cavity length.

In normal operation light output is coupled from the front of the laseradjacent to the gain section, and a small amount of light maybe takenfrom the rear of the laser adjacent the Bragg grating(s) for auxiliarypurposes such as wavelength locking. However, nothing is meant to implythat implementations with light primarily coupled out of the rear of thelaser are precluded from the scope of the invention.

1. A tuneable laser, comprising a phase change section bounded on oneside by a Bragg reflector and on the other side by a gain section, theBragg reflector reflecting at a plurality of wavelengths and comprisingat least first, second and third portions, each of which is arranged tohave current pass selectively therethrough, thereby lowering arefractive index of said portions of the Bragg reflector, to tune theportions to reflect at a lower wavelength, wherein the first portion isarranged such that it is tuneable to reflect at a lower wavelengthcorresponding to a wavelength at which the second portion reflects,thereby producing a reinforced reflection, the first and second portionsbeing tuneable such that the reinforced reflection produced by the firstand second portions is capable of being tuned to lower wavelengths,including a wavelength at which the third portion reflects.
 2. Atuneable laser as claimed in claim 1, wherein the Bragg reflectorcomprises a chirp grating formed in a material having a refractive indexvariable in response to the passage of current therethrough, and aplurality of external electrodes disposed along the length of thegrating, wherein each of said plurality of electrodes are selectivelyconnectable to a power source.
 3. A tuneable laser according to claim 1,wherein the Bragg reflector comprises a plurality of discrete gratingunits, wherein at least two of said plurality of grating units have adifferent pitch.
 4. A tuneable laser according to claim 3, wherein eachof said plurality of grating units has a different pitch, such that thegrating unit closest to the phase section has the shortest pitch, andwherein the pitch of each successive grating unit of said plurality ofgrating units extending away from the phase section is greater than thepitch of the preceding grating unit.
 5. A tuneable laser according toclaim 4, wherein each grating unit has an independently actuableelectrode.
 6. A tuneable laser according to claim 4, further comprisinga switching circuit provided to switch the current to the electrodes. 7.A tuneable laser according to claim 1, further comprising a partialreflecting mirror coupled to said gain section.
 8. A tuneable laseraccording to claim 1, wherein said Bragg reflector comprises a pluralityof discrete grating units, each of said plurality of grating unitshaving a different pitch, wherein said plurality of grating units arepositioned so as to extend away from said phase section such that thegrating unit adjacent the phase section has the shortest pitch, andwherein the pitch of each successive grating unit of said plurality ofgrating units is greater than the pitch of the immediately precedinggrating unit.
 9. A tuneable laser according to claim 1, wherein at leastone of said phase change section, Bragg reflector and gain section isformed of a semiconductor material.
 10. A tuneable laser according toclaim 1, wherein the second portion is arranged such that it is tuneableto reflect at a lower wavelength corresponding to a wavelength at whichthe third portion reflects, thereby producing a reinforced reflectionproduced by the second and third portions, the second and third portionsbeing tuneable such that their reinforced reflection is capable of beingtuned to lower wavelengths.
 11. A tuneable laser, comprising an activesection, a phase section and a Bragg reflector, wherein said Braggreflector includes at least first, second and third discrete gratingunits, each of which has a different pitch, and each of which isarranged to have current pass selectively therethrough, thereby loweringa refractive index of each grating unit such that an effectivewavelength at which each grating unit reflects is tuned to a lowerwavelength, wherein the first grating unit is arranged such that it istuneable to reflect at a lower wavelength corresponding to a wavelengthat which the second grating unit reflects, thereby producing areinforced reflection, the first and second grating units being tuneablesuch that the reinforced reflection produced by the first and secondgrating units is capable of being tuned to lower wavelengths, includinga wavelength at which the third grating unit reflects.
 12. A tuneablelaser according to claim 11, further comprising a partial reflectingmirror coupled to said active section.
 13. A tuneable laser according toclaim 11, wherein at least one of said active section, phase section,and Bragg reflector is formed of a semiconductor material.
 14. Atuneable laser according to claim 13, wherein said semiconductormaterial is a Group III–V material.
 15. A tuneable laser according toclaim 11, manufactured using electron beam writing techniques.
 16. Atuneable laser according to claim 11, manufactured using holographicphase grating plate.
 17. A tuneable laser according to claim 11, whereinthe grating unit has a length selected to produce a substantiallyuniform intensity gain response.
 18. A tuneable laser according to claim11, further comprising a Bragg grating coupled to said active section.19. A tuneable laser according to claim 11, wherein the second gratingunit is arranged such that it is tuneable to reflect at a lowerwavelength corresponding to a wavelength at which the third grating unitreflects, thereby producing a reinforced reflection produced by thesecond and third grating units, the second and third grating units beingtuneable such that their reinforced reflection is capable of being tunedto lower wavelengths.